Interpretive Summary: Climate projections, particularly longer growing seasons, have significant implications for the growth and reproduction of known agronomic weeds. One such weed is common ragweed, Ambrosia artemisiifolia. In addition to its presence in crop fields, ragweed is also a known source of allergens related to allergic rhinitis and asthma. In this study, working with the National Allergy Bureau (US) and Aerobiology Research Laboratories (Canada), we investigated the hypothesis that anthropogenic warming increases with latitude, and therefore will increase the duration of pollen shedding. Using a network of pollen counting stations for Midwestern North America, we demonstrated that the duration in common ragweed has been increasing in recent decades as a function of latitude. These latitudinal effects were associated with a delay in the first frost in fall, and a lengthening of the frost free period between spring and fall. Overall, these data are consistent with recent Intergovernmental Panel on Climate Change (IPCC) projections regarding enhanced surface warming with latitude, and confirm that such changes are likely to be associated with greater exposure times to seasonal allergens, with subsequent effects on public health. This information will be of interest to both weed scientists, public health workers, and the general public.

Technical Abstract:
A fundamental aspect regarding the impact of climate change are shifts in flowering phenology associated with milder winters, warmer seasonal air temperature and potential shifts in pollen release. Earlier floral initiation has been suggested to have a role in health impacts by contributing to greater human exposure time to allergenic pollen with a heightened risk of allergic rhinitis and related asthma. However, earlier floral initiation does not necessarily alter the temporal duration of the pollen season, and to date, no consistent continental trend in season length of pollen production has been reported with recent anthropogenic warming. Here we show, for the Midwestern region of North America, that the duration of pollen release (shedding) for Ambrosia artemisiifolia (common ragweed), a known agronomic weed, has been increasing in recent decades as a function of latitude. Latitudinal effects on increasing season length were associated primarily with a delay in first frost of the fall season and lengthening of the frost free period. Overall these data indicate a significant increase in the length of the ragweed pollen season by as much as 13-27 days at latitudes above ~44oN since 1995. This is consistent with recent Intergovernmental Panel on Climate Change (IPCC) projections regarding enhanced warming as a function of latitude, and confirms that such changes are likely to be associated with greater exposure times to seasonal allergens and subsequent effects on public health.